Oh this is easy.
A² + B² = C²
A and B are the sides.
But it's a little known fact that if those sides are 3 and 4 units, the hypotenuse is 5. Make sure to keep that in mind, because this trick works for other problems that ask what the hypotenuse is if the sides are 6 and 8, then it's 10.
The answer is 5.